Both notional and grammatical number affect agreement during language production. To explore their workings, we investigated how semantic integration, a type of conceptual relatedness, produces variations in agreement (Solomon & Pearlmutter, 2004). These agreement variations are open to competing notional and lexical-grammatical number accounts. The notional hypothesis is that changes in number agreement reflect differences in referential coherence: More coherence yields more singularity. The lexical-grammatical hypothesis is that changes in agreement arise from competition between nouns differing in grammatical number: More (...) competition yields more plurality. These hypotheses make opposing predictions about semantic integration. On the notional hypothesis, semantic integration promotes singular agreement. On the lexical-grammatical hypothesis, semantic integration promotes plural agreement. We tested these hypotheses with agreement elicitation tasks in two experiments. Both experiments supported the notional hypothesis, with semantic integration creating faster and more frequent singular agreement. This implies that referential coherence mediates the effect of semantic integration on number agreement. (shrink)